1. Field of the Invention
The present invention relates to a synchronizing control apparatus capable of synchronously rotating two rotary spindles. More particularly, it relates to a synchronizing control apparatus which is used in a machine tool, for example, a pin grinder for a crankshaft of an engine, wherein two main spindles supporting a workpiece are synchronously rotated.
2. Discussion of the Prior Art
In a prior pin grinder of this kind, left side main spindle and right side main spindle supporting a workpiece are mechanically connected each other through a synchronizing shaft in order to be synchronously rotated.
In the pin grinder, however, it is difficult to transmit the same driving power to the workpiece from the two main spindles, because transmission losses in drive trains which transmit a driving power from a drive motor to respective main spindles are different each other. Namely, the transmission loss in one of the drive trains which transmits the driving power directly to one of the main spindles is smaller than the transmission loss in the other drive train which transmits the driving power to the other main spindle via the synchronizing shaft. Further, it is difficult to perfectly synchronize the two main spindles due to backlashes of gears and the like in the drive trains. Therefore, the prior pin grinder has a difficulty in improving the dimensional accuracy and surface roughness of machined workpieces.
On the other hand, an apparatus is known, wherein master and slave rotary spindles, which are mechanically separated, are driven by a pair of servomotors each provided with a position sensor for detecting angular positions thereof. In this apparatus, the command signals, which are the same in principle, are applied to a pair of drive circuits driving respective servomotors in order to rotate the two rotary spindles synchronously. When a synchronous error, i.e., a rotational position difference between the master and slave spindles is produced, the command signal for the slave spindle is compensated based upon the synchronizing error in order to reduce the synchronizing error. The compensation of the command signal is executed whenever the command signals are output to the drive circuits.
If the electrical synchronizing apparatus is used in a pin grinder in order to rotate two main spindles, it is possible to eliminate the aforementioned problems related to the mechanical synchronization. However, the pin grinder provided with the electrical synchronizing apparatus has a problem of instability which occurs due to an interference of driving powers from the pair of servomotors. Since the pair of servomotors driving the pair of main spindles are connected with each other through a workpiece, whose both end are secured to respective main spindles, the driving power transmitted from one of the servomotors to the workpiece is transmitted to the other servomotor through the workpiece, so that the driving powers from the pair of servomotors interfere each other. Accordingly, it is required to accurately synchronize the master and slave main spindles even if the driving powers from the pair of servomotors interfere with each other. The control apparatus described above, however, does not take the driving power interference into account. Therefore, the command signal for the slave spindle is too often and too abruptly compensated so that instable conditions such as hunting motion of the main spindles continues for a long period if such instable conditions occur.